Many x-ray imaging systems employ television for image generation. Such systems include an x-ray source for propagating x-rays through a patient to emerge from the patient's body in a pattern bearing information relating to the internal structure or condition of the patient's body. An image intensifier tube is positioned opposite the source with respect to the patient, and receives the emergent pattern of x-rays incident on an input face of the intensifier tube. The image intensifier tube converts the x-ray image pattern to a visible light image corresponding to that pattern. The visible light image appears at a smaller, circular output face of the image intensifier tube.
A television camera is positioned so that its pickup tube views the circular visible image output from the image intensifier tube and converts the pattern of visible light distribution to an electrical video signal. The television camera also generates appropriate accompanying horizontal and vertical synchronization signals. The video and synchronization signals are presented together as an ensemble to a television monitor. The monitor reconverts the information carried by the signal ensemble to a second larger visible light image suitable for physician viewing for diagnostic purposes.
The circular image appearing at the intensifier tube output face is focused on the target of the television camera pickup tube by a system of lenses and mirrors which are sometimes generally referred to as an "optical cube".
"Circular blanking" is a term referring to an x-ray television system feature wherein the video image produced on the monitor is electronically blanked except for a circular area in the center of the picture. The desirability of circular blanking arises from the fact that the image produced at the output of the x-ray image intensifier tube is circular.
Radiographic imaging systems incorporating television are sometimes set up such that the television camera sweeps the intensifier tube target with its electron beam roughly tangent to the circular image focused on the target. In the absence of circular blanking, the edge of the television pickup tube target, sometimes known as the "ring", (which bears no image information) would image undesirably on the television monitor. This is at least aesthetically undesirable, and can result in distractions from normal use of the image for diagnostic purposes.
Circular blanking places the target ring, as well as other artifacts, outside the circular active imaging area, which results in a substantially noise free, or "clean", image.
Circular blanking has been achieved in prior art radiographic imaging systems by analog processing.
In analog circular blanking, the blanking is done by sensing the attainment of certain voltages by ramp or other signals of non-zero slope.
An analog circular blanking circuit is keyed to the camera vertical and horizontal synchronization signals. Such circuitry generates vertical and horizontal rate parabolas with a series of integrators.
An alternative technique generates keyed parabolas using 4-quadrant multipliers. The parabolas are then summed and applied to the input of a comparator whose input is a mixed circular blanking signal for sensor cathode blanking, i.e., blanking of the electron beam produced by the television camera pickup tube to scan the target. In addition, another comparator along with an appropriate delay constant is used to produce a mixed video circular blanking signal. A third comparator can also be used in similar fashion to generate a circular sample window required by automatic gain control circuitry.
A known type of radiographic imaging system employing television is known as DIGICON 260 and is sold by Picker International, Inc. of Cleveland, Ohio, U.S.A. A known type of television camera employed in such a system is known as the BETA X, also sold by Picker International.
Many television cameras, including the BETA X, have capability for operation in any of a multiplicity of scan formats. The BETA X, for example, can operate in any one of four different scan formats. This camera therefore requires four different circular blanking setups to provide proper circular blanking for each format.
In practice, the result is a requirement for a circuit which has 28 adjustment potentiometers. The circuit cost and complexity and difficulty of set up are apparent.
Furthermore, the possibility exists for up to six scan formats from a single television camera, implying the need for even more adjustments. The analog adjustments required result in time consuming and expensive factory and field procedures.
The analog circuitry is also susceptible to circuit drift over time and temperature, resulting in undesirable size and shape changes in the active imaging area.
It has been proposed in Mewitz U.S. Pat. No. 4,571,619 (issued Feb. 18, 1986) to employ some digital circuitry for effecting circular blanking of an x-ray image at the television monitor display. This proposal incorporates two comparators and a two's complement adder. The proposal requires a computer for its operation, and is not well suited for real time on-line processing using circular blanking. Additionally, the Mewitz circuitry, involving multiple comparators and other components, tends to be complex and expensive.
It is a general object of the present invention to provide an x-ray imaging system employing a television monitor and including improved digital blanking circuitry for defining an active image area of adjustable size and shape, symmetric about an image centerline and having flexibility for applying circular blanking to both television camera tube and to the display monitor in a real time fashion, all with circuitry which is relatively simple and compact.